1. Field Of The Invention
The present invention relates to removal of magnetically attractable impurities from suspensions of solids in a liquid vehicle by wet magnetic separation, e.g., by high intensity magnetic separation of magnetically attractable impurities from aqueous clay suspensions.
2. Description Of Related Art
In the processing of clay materials, it has been common practice in the art to utilize high intensity magnetic field separation for the removal from aqueous clay suspensions of paramagnetic (weakly magnetic) colorant impurities, e.g., iron-bearing titania and ferruginous impurities. For example, kaolin clays frequently naturally occur with such colorant impurities which impart undesired color or tint to the clay product. Both the colorant impurities and the clay particles are usually very finely divided, characteristically in the micron-size range, with much of the impurity existing as particles having an equivalent diameter of two microns or less. Magnetic separation has been found to be useful in separating the colorant impurities from slurries, i.e., aqueous suspensions, of such clay particles in order to enhance brightness of the clay.
A common type of high gradient magnetic separator apparatus employs a porous ferromagnetic matrix, e.g., stainless steel wool, which is contained in a vertically oriented cylindrical canister enclosed within an electromagnetic coil. At the upper and lower ends of the canister, ferromagnetic pole caps are disposed within the coil and a ferromagnetic return frame surrounds the coil to confine the magnetic field. Inlet and outlet openings in the canister intersecting the pole caps and return frame are provided for the aqueous clay suspension. In operation of a magnetic separator of this type, an aqueous slurry or suspension of clay containing magnetic colorant impurities is dispersed and degritted upstream of the magnetic separator and is introduced via the inlet at the bottom of the canister. The clay suspension passes through the magnetized collector which, under influence of its magnetic field, collects magnetizeable impurities in the slurry, and the resultant slurry of clay brightened by removal therefrom of the magnetically attractable impurities is withdrawn from the outlet at the top of the canister.
The electromagnetic coil is energized by a source of DC current to produce a high background field strength and set up regions of high magnetic gradient in the (e.g., steel wool) porous matrix or collector which provides numerous collection sites for the paramagnetic colorant impurities in the clay. The paramagnetic colorant impurities are collected and retained on the steel wool matrix, and, after a period of such treatment, the matrix must be cleaned of accumulated impurities. This is accomplished by discontinuing the treatment operation and flushing the matrix with water to remove the retained paramagnetic colorant impurities. The porous matrix inherently tends to retain liquids therein, including the suspension of clay solids and so a considerable quantity of clay product is flushed from the matrix by the flush water. Due to the volumes of flush water required for cleaning the matrix of the accumulated impurities, it is uneconomical to recycle the effluent flush water for processing to recover the clay solids therein, because the resultant high degree of dilution of the process stream would impose an uneconomic dewatering burden on the process. That is, the energy and equipment costs necessary to remove the added flush water from the clay suspension to attain a solids level suitable for shipping or end use of the product clay would exceed the value of the recovered clay. See, in this regard, U.S. Pat. No. 3,819,515 to J. W. Allen, and U.S. Pat. No. 4,087,358 to R. R. Oder.
The Oder patent is primarily directed to improvements in flushing collected impurities from magnetic separation matrices by applying auxiliary mechanical forces to dislodge the magnetics from the deposition medium. After process1ng a clay slurry during what is described as an initial phase of slurry feed (column 4, line 38 et seq) the magnetic separator is rinsed during a second phase of the typical operation cycle (column 4, line 54 et seq ) by rinse water flowed through it in the same direction as slurry flow (column 5, lines 1-2) while the magnetic field is still activated. The resultant rinse water-diluted slurry is withdrawn from port C of valve 52 as what the patentee states may be regarded as a "middlings" fraction, "which can be reprocessed or processed as a portion of the non-magnetics" (column 4, lines 66-68). During a third phase of the operating cycle (column 5, line 3 et seq) a high pressure flushing flow is passed through the canister in the direction opposite to that of the initial phase slurry flow and the second phase rinse flow. After this third phase of operation, treatment of the slurry is reinitiated, i.e., the first phase is repeated. At column 5, line 64 to column 6, line 10, the patentee discloses that a magnetic separator inevitably produces waste during such a cycle of operations because, after flushing, the canister "remains filled with the flush water--which then must be displaced as the processing of product is reinitiated." The patentee points out that, in turn, this displacement of flush water with product requires discarding of initial fractions of the product upon reinitiating treatment of product until complete displacement of flush water from the product, with its attendant dilution of the product, is attained. The patentee teaches to overcome this waste of product by, subsequent to flushing the magnetic separator, introducing compressed air to it to displace all of the flush water remaining in the canister and thus leaving the canister empty and ready for reinitiation of processing. However, the patentee does not teach or suggest the use of compressed air to remove retained product from the magnetic separator, but rather displaces it with rinse water to produce a "middings" fraction as described above. The patentee therefore teaches away from any suggestion of using compressed air for removal of retained product from the separator for any purpose, and entirely fails to appreciate the possibility of avoiding or reducing dilution and obtaining product yield improvement by such compressed air product removal.
U.S. Pat. Nos. 3,326,374 to G. H. Jones and 4,266,982 and 4,191,591, both to H. Bender et al, describe cleaning a magnetic separation matrix with both liquid and gaseous media.